Funds requested in this application are for the continued support and operation of the Electron Microscope Service Laboratory in the Biological Laboratories, Harvard University. This grant has continued to support the activities of numerous undergraduate students, graduate students, postdoctoral fellows and members of the faculty in their research projects using electron microscopy. For these people the service laboratory is their sole access to an electron microscope. The primary functions of the facility are to 1). provide equipment and space for electron microscopical investigations by members of the department and 2). to provide instruction in specimen preparation, electron microscopy and evaluation of results to users of the laboratory. From October 1, 1972 to July 1, 1973 the facility served forty investigators and seventeen trainees working on a wide spectrum of problems in molecular biology and the cellular and developmental biology of both plants and animals.